if your changing the oil in the diff its self (low pressure side) it wont require bleeding as its just a drop and refill job like the gearbox. if your chaging the high pressure side oil (the tank in the boot oil) then ul need to have it bled but unless the oil is stinking or uve broken in the into the high pressure side piping for sum reason, it will last yonks. its the oil in the diff itself that requires changing on a regular basis

Depends how you use the car, ive only owned mine for a year so done it for piece of mind. its recomended to replace the AYC oil every 4500 miles on a VI and gearbox oils every 18K (I think)
seeing as im not doing the mileage id say maybe once a year or at least every 2 years for the gearbox. the AYC part, at least once a year to keep the squaking away.
for the gearbox/transfer and diff i use silkolene silktran 5
for the AYC, automatic transmission fluid SP3 rated

gearbox/transferbox and rear diff are simple drop and fill items. the AYC low pressure side is the same, and this is also on the rear diff, simply drop and fill, but using ATF SP3 Spec fluid. the top resivoir you wont need to touch and will not require bleeding unless you break into the circuit.

please note that this is the case for my E6 and am guessing its the same/similar for later models. im sure the gearbox and rear diff is as above, but not 100% on the front transfer of the E8 because of the ACD which i dont have......pretty sure its not very different, again provided you drain the right compartment in the box